Dry-plate rectifier



June 26, 1956 E. NITSCHE 2,752,542

DRY-PLATE RECTIFIER Filed Sept. 21, 1951 United States DRY-PLATE RECTIFIER Application September 21, 1951, Serial N 0. 247,573 Claims priority, application Germany September 23, 1950 2 Claims. (Cl. 317-234) My invention relates to a dry-plate rectifier, more particularly to a selenium rectifier, and more especially relates to rectifiers of the type in which a plurality of rectifier plates bearing a layer of selenium and a metallic electrode coating thereon is mounted in stacked relationship with interleaved spacers, and to an improved method of producing such dry-plate rectifiers.

In prior dry-plates rectifiers of that kind the plates are stacked on a supporting rod preferably formed by a pin fitted with a surrounding insulating tube, a spring washer and an insulating washer therebeneath being interposed between successive rectifier plates to provide for a conductive connection therebetween. Hence, three parts that must be assembled are associated with each rectifier plate. In this structure the insulating washers and the spacers provide for the required mechanical stability of the column of rectifier plates transmitting the pressure required to hold the place in fixed relationship, whereas the spring washers serve the purpose to conduct the current to and from the electrode layer of each plate. In this combination it is a requirement that the spring washers exert a pressure substantially less than the pressure transferred by the spacers, because the spring washers act on the electrode layer and excessive pressure exerted on the latter would adversely affect the sensitive blocking properties existing at the boundary between the selenium and the electrode layer thereon, such blocking properties being instrumental in producing the rectifying effect. The requirement of a small contact pressure, however, is inconsistent with the requirement of minimizing the contact resistance. Such contact resistance which in the course of the last five years has been reduced from .25 ohm to .06 ohm with a rectifier plate of 112 millimeters diameter must retain its low value when provided with a protective coating of paint. Prior to the present invention attempts were made to arrive at a compromise between the contradictory requirements, such compromise necessitating severe testing of all of the parts in order to guarantee their proper functioning.

It is the object of the present invention to provide for a simplified assembly of the rectifier plates;

To reduce the electrical resistance in the direction of flow of the rectifier;

To reduce the sensitivity of the assembly rectifier to the subsequent application of a protective coating;

To provide improved plates for a dry-plate rectifier, such plates being capable of being assembled with a high pressure which, at the same time secures the current couductive relationship of successive plates;

To provide for an improved method of manufacturing a dry-plate rectifier in an inexpensive and time-saving manner resulting in uniform properties of the rectifier plates so produced.

To the accomplishment of the foregoing objects and further objects that will appear hereinafter, my invention consists in the means and the method hereinafter fully described and particularly pointed out in the claims.

The annexed drawing and the following description sets atent forth in detail a preferred embodiment of my invention which, however, illustrates but one of the various ways in which the invention may be used.

In the drawings,

Fig. 1 is a partial axial section of my novel dry-plate rectifier, and

Fig. 2 on an enlarged scale illustrates part of the section shown in Fig. 1.

On a supporting rod composed of a metal pin 1 encased by an insulating tube 2 a stack of rectifying plates is mounted with interleaved spacer bushings 7, each of the rectifier plates comprising a conductive base plate 3 preferably of sheet metal, a layer 4 thereon of a semi-conductive rectifying substance, such as selenium of the proper crystalline structure, and a metallic electrode layer 5 thereon. For the purpose of the assembly each of the rectifier plates, which are preferably of circular shape, has a central hole fitting on the supporting rod 1, 2. Suitable means not shown are provided to hold the stack together, whereby the spacer bushing 7 surrounding the supporting rod 1, 2 will exert a bearing pressure on the rectifier plate within a restricted central Zone.

For the purpose of the present invention an insulating film 6 is provided between the layer 4 and the conductive coating 5 within the area of the associated spacer 7. The film 6 which preferably is formed by a coating of lacquer serves the purpose to prevent injury to the highly sensitive current-blocking boundary 8 between the layers 4 and 5 by the assembly pressure transferred between the two successive rectifier plates through the spacer bushing 7. As will appear from Fig. 2, the film 6 prevents such boundary conditions from being set up in the area of layer 4 coated by the insulating lacquer.

Moreover, it will be noted that the layer 4 of selenium provided on each of the base plates 3 extends into close proximity of the supporting rod 1, 2 between the associated base plate 3 and the adjacent spacer 7, and that the metallic electrode layer 5 extends between the layer 4 and the spacer 7.

It must be emphasized at this point that for the purposes of the present invention the electrode layer 5 is fused on both the layer 4 and the film or coating 6.

Viewed from another aspect my invention resides in such a combination of the stack of rectifier plates having elec trodes thereon with spacers interleaved therebetween in which each spacer is so dimensioned and mounted as to exert bearing pressure but upon a central portion of the adjacent plate. Preferably, such portion of each of the plates 3 is insulated from the electrode S-fused on layer 4 of the plate.

It will be noted from Fig. 1 that the end face of bushing 7 bearing against the electrode-carrying side of the rectifier plate 3, 4, 5 is internally recessed whereby the area of contact between the bushing 7 and the rectifier plate 3, 4, 5 is restricted to an annular bearing surface which surrounds the supporting rod 1, 2 and is spaced therefrom.

In a preferred method of producing my novel rectifier, one side of each of the conductive base plates 3 is coated with a layer 4 of selenium in any suitable manner, for instance by spraying the molten selenium thereon or smearing or pressing the selenium upon the plate, or by depositing selenium thereon either by condensation of selenium vapor or by electro-deposition. Subsequently, the zone surrounding the central hole and co-operating with the spacer bushing 7 in the transfer of the assembly pressure from one rectifier plate to the other of the stack is coated with a lacquer as indicated at 6, preferably with an insulating lacquer comprising a base and a solvent both free from chemical elements other than carbon, hydrogen and oxygen. A lacquer base available on the German market by the trade name Fulvenlack dissolved in gasoline has been used successfully for the production of the film 6. Alternatively, a lacquer may be used having a silicon base. When the film 6 has dried, the electrode layer is fused thereon and on the selenium layer 4. That might be accomplished by first using a metal spraying gun in applying the layer 5 to the desired portion of layer 4 and to an outer zone of the film 6, such Zone being spaced from the central hole and having preferably an internal diameter equalling the diameter of the recess of the bushing 7 above referred to. Subsequently, the rectifier plate 3, 4, 5 is subjected in a stove to a temperature of about 210 Centigrade for a period of -40 min. This heat-treatment serves the triple purpose of completely converting the selenium into the crystalline form suited for a rectifier and of fusing the metal layer 5 and of hardening the film 6.

As a result, the electrode layer 5 will firmly adhere to both the layer 4 and the lacquer film 6, such adherence being capable to sustain all mechanical stresses set up in the assembly of the rectifier. Obviously the metal chosen for the layer 5 must be a suitable alloy having a melting point lower than that of the selenium, but sufiiciently high to effect a hardening of the lacquer 6. A metal alloy having a belting point of 170 centigrade has been found satisfactory, such as a suitable cadmium-tin alloy.

The plates 3, 4, 5, 6 so produced are mounted in a stack with the coated sides thereof facing in the same direction, and the conductive spacers 7 are interposed within the central area of each of the plates.

No particulars need be given regarding the thickness of the base plate 3, of the selenium layer 4 and of the metal layer 5, since such dimensions are well known to anyone versed in the manufacture of dry-plate rectifier-s. From the above description of a preferred embodiment of my invention it will appear that the assembly of the rectifier plates 3, 4, 5, 6 requires but the simple spacers 7 which serve the dual function of transferring the assembly pressure from one plate to another and of conducting the current between the electrode 5 of one plate and the opposed base 3 of the next plate. The spacers 7 can be produced from any suitable metal in a simple and inexpensive manner.

The dry-plate rectifier columns so assembled may be provided with any desired protective coating of paint or lacquer following the assembly. In applying such coating, the absence of spring washers liable to interfere with the coating operation is particularly advantageous. If desired, the protective coating may be applied by spraying or submerging and will effectively seal the entire dryplate rectifier from moisture and mercury.

Moreover, my invention offers the advantage that an extended operation of the rectifier at a high temperature will not result in any increase of the electrical contact resistance, Whereas in prior rectifiers such increase caused by chemical changes of the counter-electrode or the spring washer material could not always be avoided.

The outer annular zone of the film 6 covered by the Cir fused metal layer 5 is preferably made quite narrow, for instance about two millimeters or less. That offers the advantage that no blisters will form on the film 6 when the layer 5 is fused thereon, as any vapors that might be formed of solvent residues in the film 6 may reach the inner edge of the metal layer 5 and escape instead of being entrapped and forming blisters.

Having now described my invention by reference to 1 specific embodiment thereof, I wish it to be clearly understood that such embodiment is capable of numerous modifications within the scope of the appended claims.

What I claim is:

l. Dry-plate rectifier comprising a rod, a stack of rectifying plates mounted on said rod, said plates having respective coatings of semi-conductive material on one side of each plate, each of said plates having a central opening surrounding said rod and having on said coating an annular layer of fusible insulating material around said opening, an electrode layer of fused metal disposed upon said coating and said insulating layer, said metal layer forming a fusion joint with said coating and with said insulating layer, and spacer bushings on said rod interleaved with said rectifier plates. the end face of each of said bushings bearing against the electrode-carrying side of the associated one of said plates being internally recessed thereby restricting the area of contact between said bushing and said side of said plate to an annular bearing surface surrounding said rod and being peripherally spaced therefrom.

2. A dry-plate rectifier comprising a rod, a stack of rectifying plates mounted on said rod, said plates having respective coatings of semi-conductive material on one side of each plate, each of said plates having an opening surrounding said rod and having on said coating a zone of fusible insulating material around said opening, an electrode layer of fused metal disposed upon said coating and said insulating layer, said metal layer forming a fu sion joint with said coating and with said insulating layer, and spacer bushings on said rod interleaved with said rectifier plates, the end face of each of said bushings bearing against the electrode-carrying side of the associated one of said plates being internally recessed thereby restricting the area of contact between said bushing and said side of said plate to a bearing surface zone surrounding said rod and spaced therefrom.

References Cited in the file of this patent UNITED STATES PATENTS 2,314,104 Richards et al. Mar. 16, 1943 2,345,122 Herrmann Mar. 28, 1944 2,447,630 Blackburn Aug. 24, 1948 2,464,066 Addink et al. Mar. 3, 1949 2,493,643 Richards Jan. 3, 1950 2,517,602 Richards et al. Aug. 8, 1950 2,586,539 Harty Feb. 19, 1952 2,610,386 Saslaw Sept. 16, 1952 

1. DRY-PLATE RECTIFIER COMPRISING A ROD, A STACK OF RECTIFYING PLATES MOUNTED ON SAID ROD, SAID PLATES HAVING RE- 